DC motors in motor vehicles are usually driven with the aid of relays or of semiconductor switches. These are based on a series circuit comprising a semiconductor switch and a motor as an inductive load, in which the semiconductor switch can be switched on and off by means of control signals from a controller, for example a PWM controller, in other words is controllable.
If a semiconductor switch of this sort is employed in a motor vehicle, both the semiconductor switch and the motor must be protected against short-circuiting and against overload in order to prevent a catastrophic failure.
One approach to this consists in the use of what are known as “intelligent” semiconductor switches wherein, on the basis of measurements of current and/or temperature, the fault state is detected by electronics integrated into the switch, and the semiconductor switch is protected by suitable means. “Intelligent” semiconductor switches of this sort are, however, comparatively expensive, and therefore unacceptable for price-sensitive applications.
Usually, rather, standard power semiconductors, known as power MOSFETs, are used as semiconductor switches, whose protection in the event of a fault is, however, significantly more problematic, since the relevant physical magnitudes of drain current and junction temperature cannot be directly measured.
A further approach consists in connecting a fusible link in series with the controllable semiconductor switch that actuates a load circuit, in particular a motor. This, however, has the crucial disadvantage that fusible links are extremely slow, and that after an overload or a short-circuit has occurred in the load circuit, the fuse is destroyed and must be replaced.
The possibility of limiting the current through appropriate circuitry of the controllable semiconductor switch is also known. It is true that such current limitation prevents an unacceptably high current from flowing through the controllable semiconductor. In the event of an overload or of a short-circuiting in the load circuit, however, a high power loss, which can only be dissipated through suitable large cooling surfaces, occurs at the controllable semiconductor switch. In the majority of cases, however, it is not possible, for reasons of space, to attach a large cooling surface to the switching device. The current limitation circuit is consequently largely only used when only load currents of a few milliamperes occur.
A protection circuit for a controllable semiconductor switch against overload and short-circuiting in a load circuit is known from document DE 35 19 791 C2, which is incorporated by reference, wherein a measuring circuit is provided that is connected both to the load circuit and to a monitoring circuit. The current in the load circuit is here measured, for example by means of the voltage drop across a low-value resistor. If the current exceeds a set limit value, the controllable semiconductor that switches the load circuit is switched off. After a certain time the controllable semiconductor is then switched on again. This creates a measuring current which flows in the load circuit after the controllable semiconductor switch has been switched off, and is evaluated by the measuring circuit. This measuring current is sent through the load circuit until the resistance in the load circuit has reached a rated resistance. On reaching the rated resistance, the monitoring circuit is influenced, at least in respect of removing the switching off of the controllable semiconductor switch, so that it is switched on again.
It has, however, been found disadvantageous with this kind of protective circuit that the measurements cannot be made until the switch is securely fully switched on. If, however, a short-circuit is already present before the power switch is switched on, the semiconductor switch must be operated for a relatively long time in fault operation before the decision regarding deactivation can be made. In addition, in order to avoid incorrect triggering, filtering over time is usually necessary, whereby the time during which the semiconductor switch is operated in fault operation is further extended. As a result of the associated long time remaining in fault operation, the stress on the component is relatively large, and the reliability of the semiconductor switch correspondingly reduced.